The combining of two or more fluids (liquids or gases) together to form a defined mixture is fundamental to many industrial processes and commercial products. Typically, this combining is performed in discrete batches. In such a batch process, a quantity of the first fluid is added, followed by a quantity of the second fluid. These two fluids are mechanically mixed, and the resulting mixture is sampled. If necessary, additional quantities of either the first or second fluid can be further added to refine the composition of the mixture. Once the desired composition is achieved, the batch is transferred to an intermediate or end user.
This type of batching or blending process is common to many industrial segments including semiconductor processing, pharmaceutical products, biomedical products, food processing products, household products, personal care products, petroleum products, chemical products, and many other general industrial liquid products.
Batch processing, or batching, entails many drawbacks and limitations. For example, usually large tanks are required, and since this process can be time consuming, large volume batches are typically prepared at the same time. This large scale requires substantial manufacturing space, and large batch volumes dictate a relatively fixed, and inflexible manufacturing schedule. Large volumes are typically batched in order to overcome the relative imprecision of constituent fluid measurement. Large volumes help to reduce these errors as a percentage of the total batch quantity. Another drawback of batching is that the batch frequently changes its rheological or chemical properties over time. This aging effect is common to many formulations and over time it forces many adjustments to be made prior to sending the composition to the intermediate or end user. Batching can also lead to open, or partially open tanks and to fluids exposed to the atmosphere. This can lead to unwanted chemical contamination, chemical degradation and to microbial contamination.
Batching can also lead to difficulties in mixing together the fluid components in large volumes. It is often true that the components can be mixed only with difficulty and require prolonged agitation to become homogeneous. It is also well known that it is common for different levels of a large tank to have different proportionate mixtures of the fluids. It is also true that the large volumes typically committed to batching cause cleaning to be slow, laborious, and difficult to automate. Large volumes of cleaning effluents are produced, leading to issues of waste and contamination.
Because of these numerous and substantial shortcomings, and limitations, alternative means of fluid products manufacturing have been sought. One alternative method to batch processing is known as continuous blending.
Continuous blending embodies the notion of combining constituent fluids to form a fluid product only as needed or on a demand basis. Essentially, the product is made on demand and at the rate required. The rate required is typically based on the demand of the fluid filling machine packaging the liquid product.
The appeal of a continuous blending system, as distinct from a batch processing system, is clear. The ability to eliminate the large batch preparation and holding tanks leads to a small system volume, more product compounding flexibility, faster product formulation turn around, and a substantially lower capital cost. Continuous blending can also yield superior product formula accuracy, and quality, and can eliminate the barrier between fluid products processing, and fluid products packaging. Continuous blending can greatly reduce waste, cleanup time, and effluent volumes. Furthermore, the mixing is simplified and results in far more homogeneous formulations. The product aging effects are also largely eliminated. The real issue is how to build and operate a continuous blending system, with the maximum degree of accuracy, flexibility of use, and versatility of application in a broad range of commercial sectors.
Numerous designs for continuous stream blending have been proposed, originating from various liquids processing industries, particularly beverage processing and food processing. These designs have been attempts to develop and market continuous flow proportioning, or blending systems based upon ratio flow control using flow meters, and proportional-integral-derivative (PID) feedback control loops.
This is a type of feedback controller whose output, a control variable, is generally based on the error between some user-defined set point, and some measured process variable. Each element of the PID controller refers to a particular action taken on the error.                Proportional: error multiplied by a gain, Kp. This is an adjustable amplifier. In many systems, Kp is responsible for process stability; too low and the PV can drift away; too high and the PV can oscillate.        Integral: the integral of error multiplied by a gain, Ki. In many systems, Ki is responsible for driving error to zero, but, to set Ki too high, is to invite oscillation or instability or integrator windup or actuator saturation.        Derivative: the rate of change of error multiplied by a gain, Kd. In many systems, Kd is responsible for system response; too high and the PV will oscillate; too low and the PV will respond sluggishly. The designer should also note that derivative action amplifies any noise in the error signal.        
An example of this type of continuous mixer would be the “Contimix”, which was introduced by H & K Inc., of New Berlin, Wis. In general, these designs rely on regulating a continuous flow of the liquid streams using variable orifice valves or speed controlled pumps, where the flow rate signal from a flow meter, most often a Coriolis mass flow meter, is used to proportionately modulate the flow control device in order to attempt to maintain a desired ratio of flows among the streams, and where another signal representing overall system demand rate is used to proportionately modulate the summed flow of the entire system.
Several major design problems are encountered with continuous blending systems, utilizing this flow architecture. First, as the overall output of the system is increased or decreased, the rate of change capability, or response time constant of each stream, will vary one from the next. Thus, with a varying output command signal, each stream reacts at a different rate causing loss of ratio flow, and this is further aggravated by the overshoot or undershoot of each stream as a new set point is reached. Also, as each stream flow rate changes, it can perturb the flow rate of the other stream, or streams causing hunting or oscillations. These common control problems can cause serious loss of blended stream accuracy. Clearly, PID loop controllers are designed to control complex systems that are not inherently designed for stability or ease of control. They deal with the interacting, multiple dependent and independent variables of a flow stream in a non-real time, statistical way and “fight” changing parameters on an historical basis.
Still, another problem can arise when a feedback signal change causes the flow to briefly go below or above the permissible range of the flow meter generating, the feedback signal. Even with software or hardware safeties, this can occur, and as will be discussed fully further on, the requirement to maintain flow through a Coriolis mass flow meter within a defined range to achieve satisfactory accuracy is clearly demonstrable.
Perhaps, the major problem encountered with these designs, and the PID control architecture arises with the inevitable need to start and stop the flow stream system. When a stop-start event occurs, it is very difficult to bring the system back on-line with balanced, and accurate flow, and blending. This problem has been so persistent, that nearly all installed systems have resorted to the use of a surge tank, of up to several hundred gallons capacity to allow blending flow to continue during brief filler machine stoppages.
Even with the use of a surge tank, if blending flow must stop, because of a prolonged filler stoppage, upon re-start the flow streams must either be diverted until correct flow rates are reestablished, or the surge tank must be quite large to allow poorly matched flow ratios to be statistically “diluted” to prevent loss of accurate blending. Either method results in substantial waste, decreased blending accuracy, increased system complexity, and increased system volume, thus depleting the sought after advantages of continuous blending.
Therefore, there is a need in the industry for a blending system that addresses all these issues. A blending system is needed that can accommodate continuous changes in demand, while maintaining a highly precise blend accuracy. There is also a need for a blending system that integrates into an overall chemical supply and inventory scheme. A need exists for a blending system that can produce multiple blended solutions and, supply them to multiple end-users, at a high production rate, and with high resolution. A need exists for a blending system that can correct blended product that may have been temporarily stored in vessels, prior to delivery to the end-user. A need exists within the industry for a blending system that has the ability to track and confirm the chemical compositions of the initial components, intermediate blends, and final solution blends.